Rockstar has confirmed there will be gold versions of the gun you max out in multiplayer. So whatever gun you use the most will become gold.
http://xbox360.ign.com/dor/objects/14320288/red-dead-redemption/videos/reddead_trl_multimodes_42210.html
http://xbox360.ign.com/dor/objects/14320288/red-dead-redemption/videos/reddead_trl_multimodes_42210.html